A P-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Epsilon (ε) is the P-wave anisotropy parameter and equal to half the ratio of the difference between the horizontal and vertical P-wave velocities squared divided by the vertical P-wave velocity squared.
ε ≡ ½ [(C11 − C33) / C33] = ½ [(VP⊥2 − VP∥2) / VP∥2]
P-wave parameter (ε) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C11 is the horizontal P-wave modulus (perpendicular to the symmetry axis), C33 is the vertical P-wave modulus (parallel to the symmetry axis), VP⊥ is the horizontal P-wave velocity and VP∥ is the vertical P-wave velocity.
Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966.